Does acrylic sheet need to be pre-dried before vacuum forming? I thought I read somewhere that it didn't, but I keep getting bubbles that I assume are from moisture. Most of the stuff I've tried has been sitting around for a few years, but I also tried some from the local hardware store and got the same results. Any suggestions? thanks.
Are you sure you are using acrylic? Otherwise known as plexiglas or perspex with the proper chemical name of polymethylmethacrylate (say that fast five times!).
I have found a lot of times the name acrylic gets applied to almost any clear plastic sheet including polycarbonate. Polycarbonate certainly needs to be prebaked to drive offmoisture which will cause bubbles when heating for forming. To the best of my knowledge the baking is not needed for acrylic.
If it really is acrylic you have the bubbling may be caused by heating too fast. Acrylic is a poor conductor of heat and if you try heating with hot air you can overheat the surface and get bubbles from the plastic depolymerizing before the heat soaks in and softens the whole thickness. Radiant heating is preferred because the radiation penetrates slightly and the heat is absorbed in the depth of the plastic. The contact strips that are used for strip bending plastics really heat to a large extent by radiation rather than conduction.
Acrylic is hygroscopic---it absorbs water from the air---and it often does need drying, or it will bubble.
Vendors often tell you it doesn't need drying, but if it's been sitting around in a reasonably humid environment, it likely does. It usually has a protective film that slows moisture absorption, so fresh stock may be fine. Old stock is often too damp, even with the protective film.
There've been threads about this on the vacuum forming forums on hobbymolding.com and tk560.com, with pointers to instructions for making a plywood drying box. (With a lightbulb or two at the bottom to keep it warm.) A day in the hot box generally does the trick. (Or a couple of hours or so in a regular oven if you can set the temperature low enough. My oven set to 180 degrees is too hot, and the plastic bubbles when "drying".)